Re: [PATCH 3/3] btrfs: document extent mapping assumptions in checksum

[Nikolay Borisov <nborisov@suse.com>]

On 27.11.18 г. 18:00 ч., Johannes Thumshirn wrote:
> Document why map_private_extent_buffer() cannot return '1' (i.e. the map
> spans two pages) for the csum_tree_block() case.
> 
> The current algorithm for detecting a page boundary crossing in
> map_private_extent_buffer() will return a '1' *IFF* the product of the

I think the word product must be replaced with 'sum', since product
implies multiplication :)

> extent buffer's offset in the page + the offset passed in by
> csum_tree_block() and the minimal length passed in by csum_tree_block() - 1
> are bigger than PAGE_SIZE.
> 
> We always pass BTRFS_CSUM_SIZE (32) as offset and a minimal length of 32
> and the current extent buffer allocator always guarantees page aligned
> extends, so the above condition can't be true.
> 
> Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>

With that wording changed:

Reviewed-by: Nikolay Borisov <nborisov@suse.com>

> ---
>  fs/btrfs/disk-io.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 4bc270ef29b4..14d355d0cb7a 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -279,6 +279,12 @@ static int csum_tree_block(struct btrfs_fs_info *fs_info,
>  
>  	len = buf->len - offset;
>  	while (len > 0) {
> +		/*
> +		 * Note: we don't need to check for the err == 1 case here, as
> +		 * with the given combination of 'start = BTRFS_CSUM_SIZE (32)'
> +		 * and 'min_len = 32' and the currently implemented mapping
> +		 * algorithm we cannot cross a page boundary.
> +		 */
>  		err = map_private_extent_buffer(buf, offset, 32,
>  					&kaddr, &map_start, &map_len);
>  		if (err)
>